1. Field of the Invention
This invention relates to an optical disc, and more particularly to an optical disc which can reduce the fluctuation of reflectance in the land portion or groove portion thereof and can reproduce information accurately and quickly.
2. Related Background Art
Optical discs which can rapidly process high density information have been attracting attention as new computer external memories. Optical discs having a diameter of 5.25 inches include the write once type which is capable of writing information only once and the magneto-optical type which is capable of rewriting information. Also, optical discs having a diameter of 3.5 inches include the magneto-optical type, the ROM type which is exclusively for use for reproduction, and the partial ROM type in which the magneto-optical type and the ROM type are combined, and these are standardized by the ISO standards and therefore are expected to spread more widely in the market in the future.
In these optical discs, a guide for directing a laser spot from the pickup of a recording-reproducing apparatus along information, i.e. for tracking, is formed spirally or concentrically from the inner periphery toward the outer periphery of the disc in the form of a concave or convex groove.
The structure of the optical disc will hereinafter be described in greater detail with reference to FIG. 1 of the accompanying drawings. A portion which is concave when viewed from a pickup 4, that is, which is far from the pickup 4, is called a land 1, and a portion which is convex when viewed from the pickup 4, that is, which is near to the pickup 4, is called a groove 2. Also, the distance from the center of a land to the center of an adjacent land is called a track pitch 3. In this structure, information can be written into either the land 1 or the groove 2, but many optical discs adopt the land recording type in which information is written into the land portion.
The grooves of optical discs of the land recording type which are now put into practical use include two kinds, i.e., U-shaped grooves as shown in FIG. 2 of the accompanying drawings, and V-shaped grooves as shown in FIG. 3 of the accompanying drawings. In the case of the U-shaped groove, the groove width W thereof is defined as W=(Wtop+Wbottom)/2 when the width at the top of the groove is Wtop (5) and the width at the bottom of the groove is Wbottom (6). In the case of the V-shaped groove, the groove width W thereof is defined as W=Wtop (5). In any case, the land width is defined by "track pitch P-groove width W". That is, in the U-shaped groove, the land width=P-(Wtop+Wbottom)/2, and in the V-shaped groove, the land width=P-Wtop.
It has generally been the case that the groove width W is 0.4 .mu.m-0.6 .mu.m and the track pitch is 1.6 .mu.m, but recently, there have been reported optical discs in which the track pitch is 1.4 .mu.m or 1.2 .mu.m to realize a narrow track pitch in order to record higher density of information.
In the conventional optical discs, particularly when the track pitch is smaller than 1.6 .mu.m, the reflectance of the land portion (the quantity of light returning to a signal detector (photosensor) in the pickup when the laser spot is positioned on the center of the land) unavoidably fluctuates due to the minute irregularity of the groove width, groove depth and track pitch attributable to the fluctuations of conditions during the manufacture of the optical discs and the minute irregularity of the reflectances of recording film and reflecting film. This has led to the undesirable result that the error of the writing or reading of information occurs or a tracking error occurs during high-speed search or access, thus presenting a significant problem in the narrowing of the track pitch of the optical disc.